It has long been desirable to be able to control the surface characteristics of textiles and fabrics including nonwoven materials. A particularly important characteristic is whether a material readily absorbs or repels water at its surface. For fibers used to make nonwoven materials for use in personal care products and for many other uses, the relative hydrophobicity or hydrophilicity of the fiber itself, and the material made from it, is of great importance in determining where and how they can be used.
When brought into contact with the surface of a material, water prefers to wet some surfaces and prefers to bead on others. A surface can be classified as hydrophilic, with a water contact angle less than 90°, or hydrophobic, with a water contact angle greater than 90°, based on the shape that a drop of water assumes when placed on that surface.
Fabric water repellency and breathability have been studied for several decades (A. W. Adamson, Physical Chemistry of Surfaces, Second Edition, Wiley, 1967, Chapters VII and X). A nonwoven web of fibers can be modeled as a bundle of cylindrical pores (capillaries) of radius r. The fluid pressure required to penetrate the interfiber pores of a nonwoven web can be approximated from Laplace's equation for the penetration of a fluid into a tube as follows: P=(2 γ cos θ)/r
where:
P=pressure required to push fluid through the tube;
γ=fluid surface tension;
θ=advancing contact angle; and
r=pore radius.
See Dutkiewicz, J., Nonwoven Structures for Absorption of Body Fluids, sub-chapter 2.1. Basic Structural Properties of Absorbent Networks, pages 7-37 (published by Edana, Brussels, Belgium) (2003). This equation can be used to describe web wetting (θ<90°, P is positive) or web water repellency (θ>90°, P is negative). In the case of water repellency, the fluid will not wet the web unless a pressure of P is applied to push the fluid into the web.
From the equation, barrier quality is predicted to be enhanced by increasing the contact angle with a water-repellent finish. In other words, the pores of the web should be rendered as hydrophobic as possible.
Apparent contact angles can be increased by surface roughness on the macroscale and microscale. Application of a waterproofing agent that causes microscopic pore surface roughness will lead to an increase in apparent contact angle, thus improving barrier quality.
From the equation, barrier quality is predicted to be enhanced by reducing the size of the interfiber pores. Ideally, the web should be as strong as possible. As pressure builds, weakness in the web will cause deformation, and deformation increases r, thus lowering pressure P. Web strength can be enhanced, for example, by increasing the amount of binder in the web.
The size of interfiber pores in a fibrous web is determined by the fiber size and the density or extent of compaction of the web. Increasing the density of the web can reduce the size of interfiber pores, or using smaller diameter fibers at the same density can reduce them. Smaller fibers pack together more efficiently in a densified web, resulting in smaller interfiber pores. From the equation, using smaller fibers serves to decrease r, thus raising pressure P.
Filler material can be added to an absorbent material to reduce the size of interfiber pores. From the equation, the addition of filler also serves to decrease r, thus raising pressure P.
From the equation, hydrophobicity and barrier quality is predicted to be directly proportional to the fluid surface tension. The barrier treatment should be as durable as possible. Any additives in the barrier treatment that will dissolve in the fluid coming in contact with the surface of the material will likely lower its surface tension, thus lowering pressure P.